1. Field of the Invention
The present invention relates generally to memory systems, and more particularly, to methods, systems and apparatus for phase change memory cells.
2. Description of the Related Art
The general concept of utilizing electrically writable and erasable phase change materials (i.e., materials that can be electrically switched between generally amorphous and generally crystalline states) for electronic memory applications is well known in the art. A typical phase change material is a material that has two general states: a generally amorphous state and a generally crystalline state. The phase change material can include one or more chalcogenide compounds that at least partly include one or more of the following materials: Te, Se, Sb, Ni, and Ge and various combinations thereof. The typical phase change material can be switched from one state to the other by passing an electrical current through the phase change material to cause it to change states. Typically in the first state (e.g., amorphous state), the phase change material has a relatively high resistance and in the second state (e.g., the crystalline state), the phase change material has a relatively low resistance. As the state of the phase change material can only be changed by application of energy, then the phase change material is generally non-volatile in that it does not require energy to maintain it's current state. Further, because the resistance of the phase change material varies with the state, then the phase change material can be reliably used to store binary data such as may be used for a memory cell in a computer or other binary data storage usage.
A traditional phase change memory cell structure includes a transistor or diode as a steering element, in series with the phase change memory element. FIG. 1A shows a prior art phase change memory cell array 100. Four memory cells 110A-110D are included in the memory cell array 100. Each memory cell 110A includes a transistor 102, as a steering element, and one phase change device 104 as a memory element. FIG. 1B shows another prior art phase change memory cell array 150. Four memory cells 160A-160D are included in the memory cell array 150. A diode 152 is used as the steering element instead of the transistor 102. The need of a steering element increases the overall size, cost, complexity and power consumption of a phase change memory cell.
A generally small contact structure at one end of the phase change device 104 is typically preferred. A smaller contact structure can yield a smaller overall device size, thereby allowing a more densely packed integrated circuit of such phase change memory cells. More importantly, however, a smaller contact area can also yield a lower switching current and consequently a lower overall switching power consumption. However, the manufacturing processes required to form such a small contact structure is very complicated and therefore has a relatively high process cost.
In view of the foregoing, there is a need for a simpler, smaller, more power efficient and more easily manufactured phase change device that can be more efficiently used such as in a phase change memory cell.